1. Field of the Invention
The present invention relates to a working vehicle having a grass cutting unit vertically movable relative to a vehicle body, and particularly to a raising and lowering control of the grass cutting unit.
2. Description of the Prior Art
Conventionally, a lawn mower often includes a hydraulic control device for raising and lowering a grass cutting unit, and a control device for controlling a PTO clutch to make or break torque transmission to the grass cutting unit. Vertical movement of the grass cutting unit and rotation of cutting blades are controlled by manually operating the two control devices during a grass cutting operation.
When such a lawn mower is turned around at an end of a plot of ground on which a grass cutting operation is carried out, the controls for vertically moving the grass cutting unit and operating the PTO clutch are carried out along the maneuvering of the lawn mower. Considerable skill is required to effect such controls smoothly and promptly. An error in the control sequence tends to result in uncut patches or an unsatisfactory finish.
To solve this problem, the PTO clutch may be linked with a control valve of a hydraulic cylinder from vertically moving the grass cutting unit, to improve efficiency of the grass cutting operation by operating a single control device to effect both vertical movement of the grass cutting unit and operation of the PTO clutch. However, a great operating force is required where an interlock is made between push-pulll movement of the control valve and a movement transmission member such as rods or wiring for operation the PTO clutch. Moreover, means is required for absorbing and adjusting a difference in mechanical stroke between the two control systems, which tends to complicate the construction. The control valve may be of the electromagnetic type, but this in practice has a disadvantage in terms of cost in that an electric control device is required in addition to an expensive hydraulic control system including pumps, control valves, piping and hydraulic cylinders.
Further, the grass cutting unit usually is designed to be lowered to variable positions to adjust cutting height in an operative condition. The grass cutting unit is movable to a fixed upward position for an inoperative or contained condition. It has been found that this aspect produces certain inconveniences depending on the particular use of the lawn mower. The grass cutting unit is raised to a high upward position during a run on a road, to avoid contact with road surfaces. However, when running on a sloped terrain such as on a golf course, the lawn mower has an elevated center of gravity because the grass cutting unit is raised to the high position. Consequently, the lawn mower must run in an unstable state compared to running during grass cutting operation.